Antiknock agent

Anti -knock preparations, sometimes also called " octane boosters" are additives to petrol, which reduce the knocking tendency of the gasoline engine by increasing the octane number. They are used to prevent uncontrolled spontaneous combustion (knocking ) of fuel in the cylinder. The knock resistance is measured by the octane number (RON = research octane number ).

Origin of knock

The uncontrolled combustion of the fuel-air mixture takes place, because during the compression in the cylinder temperatures already produced which are sufficiently high to break down the molecules into radicals before the spark initiates a controlled ignition.

This happens especially in the n- alkanes whose carbon atoms are arranged to form a chain. The resulting in-cylinder radicals combine readily with oxygen molecules. Because the radicals are so reactive, is this not even the heat of the engine needed.

The knock is the stronger, the higher the proportion of hydrogen radicals. Therefore, to be reduced by anti-knock agent the proportion of free hydrogen radicals.

Operation of the anti-knock agent

By the uncontrolled combustion of the engine can be ruined. Such reactions may be prevented by the addition of anti-knock compounds such as alcohol (ethanol) with which the material reacts in place of oxygen. Here, a less strong exothermic reaction takes place, so that the motor is not so badly damaged. The resulting ethane is burned together with the fuel.

History

The anti-knock gasoline Super Motalin was in the second half of the 1920s in Germany provided with the additive of iron pentacarbonyl. This additive was also included in the so-called Motylpatronen ( a mixture of iron carbonyls ). Tetraethyl lead ( " leaded gasoline " ) was used as a common anti-knock agent since 1924 in the U.S. and from the mid-1930s in Germany. This was replaced with the introduction of unleaded gasoline by methyl tert-butyl ether (MTBE ) and ethyl tert -butyl ether ( ETBE), since lead has damaged the car exhaust catalyst, is poisonous and dangerous for the environment.

However, as the difference in this case is to be noted that the organolead compounds act at low concentrations (~ ≤ 1 g / liter), while MTBE is added in the percentage amounts. Economically considered energy is thus the "alternative anti-knock agent " even for relevant parts of an energy carrier.

In earlier times could therefore also inferior petroleum distillates ( from engines view) bolstered with the lead additives and used as fuel. Later, the method of petrochemical gasoline reforming, bringing the desired octane numbers were achieved without lead improved. However, this means that the entire hydrocarbon fuels is changed more or less strong in its chemical structure. This is associated with effort, even on the energetic level.

A target product here is the MTBE. Another " anti-knock " class of materials are the aromatics. A little dubious representative is toluene here. The benzene is teratogenic and toxic, so its content is limited (which is also identified on the certificate at the pump ).